Patients with systemic lupus erythematosus are unusually susceptible to infection both because of their disease and because of the therapy for it. We surveyed the medical records of 571 patients with SLE seen at NIH, the records of patients with seven other inflammatory diseases for cases of bacterial endocarditis, and surveyed the records of all patients with endocarditis seen at NIH. The incidence of endocarditis was much higher in the lupus patients than in the patients with other diseases. Murmurs, valvular abnormalities, and dental procedures were predisposing in some but not all patients. In a random survey of 20 current lupus patients, no previously undetected valvular abnormalities were found by echocardiography.